Dengue viruses are mosquito-borne pathogens of the genes Flavivirus (family Flaviviridae). Four serotypes of dengue viruses (often abbreviated “DEN”) have been identified, including dengue-1, dengue-2, dengue-3 and dengue-4 (DEN-1 to DEN-4). The flavivirus genome is a single-stranded, positive-sense RNA approximately 11 kb in length, containing a 5′-noncoding region (5′NC); a coding region encoding the viral structural proteins; five nonstructural proteins, designated NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5; and a 3′-noncoding region (3′NC). The viral structural proteins include the capsid, premembrane/membrane, and envelope. The structural and nonstructural proteins are translated as a single polyprotein. The polyprotein is then processed by cellular and viral proteases.
Transmitted by Aedes aegypti mosquitoes to humans in tropical and subtropical regions of the world, dengue viruses cause millions of cases of disease every year, ranging from dengue fever to the often fatal dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). Secondary infection of humans with a heterologous serotype of DEN virus may induce an immunopathological response and is considered a possible risk factor for DHF/DSS. Therefore, the need exists for development of a vaccine that confers simultaneous protection against all dengue virus strains.
Since eradication of Aedes aegypti mosquitoes appears to be practically infeasible, development of safe, effective vaccines against all four serotypes of dengue virus is a World Health Organization priority. However, no approved, effective vaccine against any of the dengue virus strains is currently available. It has been demonstrated that serial passage of wild-type flaviviruses in various cell cultures, such as primary dog kidney (PDK) cells, produces virus variants that have reduced virulence, retain immunogenicity and produce no untoward clinical symptoms.
Live, attenuated dengue viruses of all four serotypes have been developed at Mahidol University in Thailand by passaging the wild-type viruses in cell culture. These are currently the most promising live, attenuated vaccine candidates for immunization against dengue virus infection and/or disease. These vaccine candidates have been designated by a combination of their dengue serotype, the cell line through which they were passaged and the number of times they were passaged. Thus, a dengue serotype 1 wild: type virus passaged in PDK cells 13 times is designated as DEN-1 PDK-13 virus (nucleotide sequence, SEQ ID NO:3; amino acid sequence, SEQ ID NO:4). The other vaccine candidates are DEN-2 PDK-53 (nucleotide sequence, SEQ ID NO:15; amino acid sequence, SEQ ID NO:16), DEN-3 PGMK-30/FRhL-3 (thirty passages in primary green monkey kidney cells, followed by three passages in fetal rhesus lung cells) (nucleotide sequence, SEQ ID NO:21; amino acid sequence, SEQ ID NO:22) and DEN-4 PDK-48 (nucleotide sequence, SEQ ID NO:25; amino acid sequence, SEQ ID NO:26). These four candidate vaccine viruses were derived by tissue culture passage of wild-type parental DEN-1 16007 (nucleotide sequence, SEQ ID NO:1; amino acid sequence, SEQ ID NO:2), DEN-2 16681 (nucleotide sequence, SEQ ID NO:13; amino acid sequence, SEQ ID NO:14), DEN-3 16562 (nucleotide sequence, SEQ ID NO:19; amino acid sequence, SEQ ID NO:20) and DEN-4 1036 (nucleotide sequence, SEQ ID NO:23; amino acid sequence, SEQ ID NO:24) viruses, respectively.
Preliminary human clinical trials with these attenuated viruses have indicated that DEN-2 PDK-53 has the lowest infectious dose (50% minimal infectious dose of 5 plaque forming units or PFU) in humans, is strongly immunogenic, and produces no unacceptable clinical symptoms. The DEN-1 PDK-13, DEN-3 PGMK-30/FRhL-3 and DEN-4 PDK-48 vaccine virus candidates have higher 50% minimal infectious doses of 10,000, 3500, and 150 PFU, respectively, in humans. The higher infectious doses required for the latter three vaccine candidates raises concerns regarding the relative efficacy of each serotype component in a tetravalent dengue virus vaccine. Although only one immunization with monovalent DEN-2 PDK-53 virus or DEN-4 PDK-48 virus was required to achieve 100% seroconversion in human subjects, a booster was needed to achieve the same seroconversion rate for DEN-1 PDK-13 and DEN-3 PG1V1K-30/FRhL-3 viruses, which have the two highest infectious doses for humans.
The DEN-2 PDK-53 virus vaccine candidate, henceforth abbreviated PDK-53, has several measurable biological markers associated with attenuation, including temperature sensitivity, small plaque size, decreased replication in mosquito C6/36 cell culture, decreased replication in intact mosquitoes, loss of neurovirulence for suckling mice and decreased incidence of viremia in monkeys. Clinical trials of the candidate PDK-53 vaccine have demonstrated its safety and immunogenicity in humans. Furthermore, the PDK-53 vaccine induces dengue virus-specific T-cell memory responses in human vaccine recipients.
Except for DEN-2 PDK-53 virus, the number and identity of the genetic mutations that accrued during multiple passages in cell culture and that are associated with the attenuated phenotypes of the vaccine candidates are unknown. Neither the relative contributions of such attenuation-associated mutations to the actual mechanism of attenuation, nor the potential for reverse mutations to revert any of the vaccine candidates to the virulent biological phenotype of the wild-type dengue virus are known for any of these four vaccine candidates. An understanding of the attenuation markers of a vaccine candidate is critical for the prediction of its stability and safety.
Accordingly, there is a need for avirulent, yet immunogenic, dengue viruses to be used in the development of dengue virus vaccines to confer protection against one or more dengue virus serotypes. What would be ideal is a vaccine that would simultaneously protect an individual against several virulent strains of this potentially dangerous family (Flaviviridae) of viruses. Therefore, a tetravalent vaccine that can be used to immunize an individual against all four dengue serotypes is particularly needed.